In the fitting of a hearing aid, it is important to measure the occlusion of the actual earmold to determine the amount of leakage and whether there are any occlusion effects associated with the earmold. Measurement of the occlusion effect is performed by comparing sound pressure levels generated by the wearer's voice, for example, within the ear canal without the earmold with the sound pressure levels generated within the ear canal with the earmold in place. These sound pressure differences provide an indication of the degree of occlusion provided by the earmold. Similarly, measurement of earmold leakage can be made inside the ear canal (behind the earmold) with the sound level generated near the ear by an external sound source. The term "degree of occlusion" shall hereinafter be used to refer to the degree to which an occluding object, such as an earmold, prevents leakage and/or inhibits occlusion effects. In most instances, the measured difference between the sound pressure levels described above provide a quantitative measure of the degree of occlusion of an occluding object.
The failure of an earmold to properly seal and fit within the ear canal of a subject can result in unwanted repercussions. Without a close fit between the earmold and the ear canal wall, excessive leakage may result. This excessive leakage may result in annoying feedback that renders the hearing aid useless. With respect to occlusion effects, the bone conduction of the wearer's voice may be amplified when the ear is occluded. The hearing aid wearer's own voice is thus heard as being loud and distorted. This effect was noted by J. Zwislocki in his article entitled "Acoustic Attenuation Between The Ears", J. Acous. Soc. Amer. 25:752-759 (1953). The value of deeply sealed earmolds to eliminate this effect is discussed in an article by Mead Killion entitled "The `Hollow Voice`Occlusion Effect" Hearing Aid Fitting--Theoretical and Practical Views, CH. III, pp. 231-242 (1988). It is therefore important in some fittings to ensure that the earmold seal extends far enough into the ear canal to eliminate this occlusion effect.
Testing of the quality of the earmold has heretofore been performed using expensive and bulky equipment such as a Frye 6500 available from Frye Electronics. Measurements with the Frye 6500 are performed sequentially. First, the subject being tested is provided with a sound stimulus without the earmold inserted. The sound stimulus is external when measuring leakage and self generated by the subject's own voice when measuring occlusion effects. The sound pressure level of the sound stimulus is measured by the Frye 6500 using a microphone probe that is positioned in the ear canal. The measured signals are stored and displayed as a frequency domain graph on a video display. After the reference measurements are made, the subject is provided with a further sound stimulus with the earmold inserted. The Frye 6500 again measures the sound pressure level within the ear canal as detected by the microphone probe. The frequency domain response of the sound detected in the ear canal with the earmold in place is then displayed on the video screen. Both frequency domain graphs are provided on the video display simultaneously to provide a visual comparison of the difference in sound pressure levels within the ear canal.
The Frye 6500 is a useful but complicated device which is often beyond the financial means of many audiologists. Additionally, it is a complex device that is often difficult to use without proper training. As such, audiologists must expend a substantial amount of otherwise valuable time to ensure that they are properly trained in use of the device. Further, the device is large and is not easily transported between examining rooms in the audiologist's offices.